Many pearlescent or nacreous pigments, also known as color effect materials (CEM's) are based on the use of a laminar substrate such as mica which have been coated with a metal oxide layer. As a result of reflection and refraction of light, these pigments exhibit pearl-like luster and depending on the thickness of the metal oxide layer, they can also exhibit interference color effects. The pearlescent pigments encountered on a commercial basis are most often either titanium dioxide-coated mica or iron oxide-coated mica pearlescent pigments. Both types are well known in the art.
The overcoating of the titanium dioxide-coated or iron oxide-coated mica pearlescent pigment is also well known in the art. See, for example, U.S. Pat. Nos. 3,087,828; 3,087,829; 3,711,308; 3,874,890; 4,146,403; 4,886,100; and 5,137,575.
Color effect material or special-effect pigments are employed in numerous fields in industry, especially in the sector of automotive finishes, in decorative coating, in plastics, in paints, in printing inks, cosmetics and personal care formulations.
Color effect material which exhibit an angle-dependent color change between two or more interference colors have a play of color which makes them of particular interest for automotive finishes and in connection with counterfeit-protected documents of value. CEM's of this kind on the basis of multiply coated platelet-shaped substrates are known.
CEM's consist generally of platelet-shaped substrates with a thickness of from 200 to 10000 nm which are coated with highly refractive metal oxides or metal oxide mixtures with a thickness of from 50 to 300 nm. The optical properties of these pigments are critically determined by the refractive index of the metal oxide layer. In addition to the possibility of using chemical vapor deposition (CVD) or physical vapor deposition (PVD) techniques to prepare metal oxide layers having high densities and refractive indices that lie close to the optimum, the deposition of metal oxides on finely divided, platelet-shaped substrates is frequently accomplished by titrating aqueous, usually acidic metal salt solutions against sodium hydroxide solution in the presence of a substrate, as described, for example, in DE 14 67 468 and DE 20 09 566.
JP H7-759 discloses a multilayer interference pigment with metallic luster, for which a substrate is coated with alternate layers of titanium dioxide and silicon dioxide. The substrate comprises flakes of aluminum, gold or silver, or of mica or glass, with a coating of metals. The depth effect which is characteristic of and desired for interference pigments, however, cannot be generated. This is because of the total reflection of the light at the metal layer which forms the core. Consequently, the interference effect remains limited to the layers which are located on the metal layer. Furthermore, the lack of transparency of the substrate greatly restricts the diverse possibilities for combination with further pigments in applications-related formulations.
U.S. Pat. No. 6,596,070 provides an essentially transparent interference pigment which is based on multiply coated, platelet-shaped substrates and comprises a particular arrangement of optically functional layers by means of which particular optical effects are achieved. The invention provides interference pigments on the basis of multiply coated, platelet-shaped substrates which comprise at least one layer sequence comprising (A) a coating having a refractive index n≧2.0, (B) a colorless coating having a refractive index n≦1.8, and (C) a coating of high refractive index, and, if desired, (D) an external protective layer.
Copper chromite, also known as Pigment Black 28, is a black material. When it is coated in a sufficient quantity on a laminar or platy substrate such as mica, the coating is opaque. In contrast, pearlescent pigments are based on the fact that there is reflection at the surface of the coating and also a transmission through the coating to the next interface, and reflection at that next interface.
In commonly assigned U.S. Pat. No. 6,616,745, it had been discovered that when copper chromite is coated onto a metal oxide-coated, e.g. a titanium dioxide- or iron oxide-coated mica pearlescent pigment, there is color advancement of the interference color and an increase in the opacity of the pigment. It had also been found that when the layer is thick enough, the color progresses to the next shade and 100% opacity could be achieved at lower pigment loads.
It had also been found that copper chromite can be coated onto a platy substrate, e.g., mica, kaolin, etc., and then coated with a metal oxide layer which produces colored black pigments. The initial layer of copper chromite can be of sufficient quantity to be opaque (about 45% CuCr2O4 when the substrate is mica) or semitransparent. Commonly assigned U.S. Pat. No. 6,616,745 is hereby incorporated by reference.